The present invention relates, in general, to a method and apparatus for tracking and guiding the drilling of generally horizontal boreholes below the earth's surface, and more particularly to an improved system and apparatus for tracking a borehole being drilled generally horizontally under an obstacle such as a river, where access to the surface of the ground immediately above the borehole is difficult. Measurements of borehole location and direction are made for use in guiding the borehole to a specified location.
Horizontal directional drilling techniques are well known, and have long been used to drill boreholes which cross under areas where trenching is not permitted or is impractical. For example, such techniques are used to drill boreholes under manmade or natural obstacles such as rivers, lakes, or other bodies of water, under highways, airport runways, housing developments or the like. These boreholes may be used to position pipelines, underground transmission lines, communications lines such as optical fibers and other utilities, for example, and often must be drilled within defined areas, must travel long distances, and must exit the ground at predetermined locations. The borehole typically is tunneled from an entry point on the earth's surface at the near side of an obstacle, travels under the obstacle, and exits the ground at a predetermined location on its far side. In drilling such boreholes it is important to maintain them on a carefully controlled track following a prescribed drilling path, or proposal, for often the borehole must remain within a right of way as it passes under the obstacle and its entry and exit points on opposite sides of the obstacle must often be within precisely defined areas.
Conventional directional drilling apparatus for drilling such boreholes commonly incorporates a steering tool which measures the borehole inclination, magnetic azimuth, and tool roll angle with respect to the earth's gravity and magnetic field at each station where measurements are made. The borehole coordinates are computed and tabulated from these steering tool data as a function of the measured distance along the borehole, which may be referred to as the measured depth of the steering tool. These borehole coordinates suffer from serious cumulative effects produced by inclination and azimuth determinations made at spaced locations along the borehole, and by the lateral errors generated by conventional borehole surveying techniques. The inherent imprecision of these techniques is the reason for turning to electromagnetic methods for directly determining drill bit location.
Prior systems such as those illustrated in U.S. Pat. Nos. 4,875,014 and 3,712,391 provide guidance for the drilling of boreholes, but in some circumstances present problems to the user since they require access to the land above the path to be followed by the borehole. These systems utilize surface grids or other guidance systems on the earth's surface, but the access they require often is not available.
U.S. Pat. Nos. 5,513,710 and 6,626,252 overcome the foregoing problem by providing drilling guidance methods and systems for drilling boreholes under rivers and under obstacles, the '710 patent utilizing a direct current powered solenoid at a known location with respect to the target exit for the borehole, and the '252 patent utilizing two horizontal AC solenoids near a borehole path on the surface of the earth at the far side of the obstacle.
The foregoing systems require precise location and orientation of the solenoids used to provide the magnetic fields, and this can be an inconvenience in some circumstances and impossible in others, where there may not be access to an appropriate location for the solenoids or where there is not sufficient time to carry out the required orientation procedure. Thus, there is a need to provide a simple, yet accurate system for detecting and tracking the drill stem used to produce an underground borehole, where a magnetic field source such as a solenoid can be deployed in a body of water, for example, above the path of the borehole being drilled, and where it is not necessary to determine the orientation of the source.